In a dual band mobile phone the operational bandwidth of the antenna presents a difficult problem. By example, such a mobile phone may need to be operable in the 800 MHz band (e.g., analog AMPS) at one time, and then in the 1.9 GHz band (e.g., digital PCN) at another time. Preferably, a single antenna is used for both bands. However, due to the large difference in operational frequencies conventional dual band mobile phones typically provide a separate receiver front end and a separate transmitter power amplifier section for each band.
FIG. 1 illustrates one such conventional construction for a dual band mobile phone. A single antenna system 1 is connected to a Band 1 receiver front end and transmitter power amplifier block 2, and is also connected to a Band 2 receiver front end and transmitter power amplifier block 3. Blocks 2 and 3 are both connected to a common, further block 4 that contains the receiver IF circuits, frequency synthesizer(s), modulators, demodulators, etc. Not shown in FIG. 1 are the remaining portions of the mobile phone, such as the keypad, display, user interface controller, etc.
In some dual band phones, such as a dual band phone that includes a digital TDMA portion, the TDMA transmitter and receiver sections are connected to the antenna 1 through a duplex filter 5, as is illustrated in FIG. 2. In this case the duplex filter 5 is connected to an input of a receiver amplifier 6, which has an output connected to a bandpass filter 7 and a first IF mixer 8, while also being connected to the output of a transmitter power amplifier 9.
In conventional practice the required band switching has been accomplished with a mechanical relay or switch 10, as is shown in FIG. 3. In FIG. 3 the circuitry shown in FIG. 2 is duplicated for each band, with the Band 2 circuits being designated with prime symbols ('). At any given time the switch 10 connects either the Band 1 or the Band 2 transceiver circuitry to the antenna 1.
However, this approach has at least two significant disadvantages. First, the use of a mechanical switch has the disadvantage of requiring a large and bulky component with a slow switching time. To overcome this problem the mechanical switch can be replaced with an electronic switch, such as a FET or a pin diode. However, the use of an electronic switch is disadvantageous in that it introduces a significant insertion loss into the RF path. A typical insertion loss for an electronic switch is on the order of 0.5 dB. This insertion loss must be compensated for by using a higher transmitter power which, in turn, increases the power consumption and reduces the battery life of the mobile phone. Furthermore, if operation in one of the bands is required to be a duplex analog mode, the antenna switching circuitry must exhibit a high degree of linearity to avoid spurious responses generated by the transmitter signal within the antenna switch 10. Also, the receiver sensitivity is impaired since the switch insertion loss also degrades the receiver noise figure.
One benefit of the approach shown in FIG. 3 is that it provides good isolation between band filters (in this case the duplex filters, or duplexers, 5 and 5'), which thereby enables an optimum performance on both bands. However, the high insertion loss and required linearity can overshadow the benefit of this arrangement, and make its use undesirable for many applications.
Reference can also be made to FIG. 4 for illustrating a conventional approach for implementing antenna switching in a dual band mobile phone, when an external antenna connector 1a is required. That is, the antenna 1 may be an integral or master antenna that is provided as part of the mobile phone, while antenna connector 1a is also provided to enable the mobile phone to be connected to a second, external antenna. In addition to the Band 1 and Band 2 transceiver circuitry shown in FIG. 3, a second antenna switch 11 is connected in series with the band switch 10, and suitable antenna switch control lines are provided from a controller (not shown) of the mobile phone to control the state of the antenna switches 10 and 11. Assuming that the second switch 11 is also an electronic switch, it can be appreciated that the total insertion loss can be doubled over the FIG. 3 configuration. This in turn compounds at least the power consumption and receiver sensitivity problems that were described above with respect to FIG. 3.
FIG. 9 illustrates a conventional antenna/external antenna switching arrangement for a single band digital (e.g., TDMA) mobile phone. The first antenna switch 10 functions as a transmit/receive switch for the receiver and transmitter, and is coupled to a receive bandpass filter 5a and a transmit bandpass filter 5b. The second antenna switch 11 is employed for switching between the antenna 1 and the external antenna connector 1a. As in the embodiment of FIG. 4, a significant disadvantage of this technique is the doubling of the insertion loss due to the two serially coupled antenna switches 10 and 11.